This invention pertains to apparatus required to provide precision mechanical rotation of a turntable in a vacuum atmosphere and more particularly, to precision turntable rotation suitable for electron beam recording of video discs.
A system for recording and playback of video information has been described in a copending U.S. application Ser. No. 126,772 of Jon K. Clemens filed Mar. 22, 1971. As described in the Clemens application, a lacquered surface is deposited on a thick aluminum disc and a continuous spiral groove is cut into the lacquered surface. A first nickel replica is then made of the grooved lacquer surface by depositing nickel over such surface and thereafter separating the nickel coating from the lacquer. This first nickel replica is a negative reproduction of the original grooved lacquer. A second nickel replica is made of the first replica to form a metal disc that is a positive reproduction of the original grooved lacquer. A uniform coating of energy sensitive material such as a photoresist is thereafter applied to the second nickel replica.
The photoresist-coated replica is then exposed by a video-signal-modulated beam of a scanning electron microscope which provides exposure of the photoresist in the groove region of the disc in correspondence to the video signal information. The photoresist is then developed and the exposed portions of photoresist are removed to form a topography in the groove corresponding to the video signal information. A nickel replication is made of the resultant disc and this replication is utilized to stamp or emboss vinyl records by techniques known in the audio recording art. The vinyl replica is then metalized to make the surface conducting and the metalization is thereafter coated with a dielectric. In playing back the recorded information, a stylus is caused to ride in the dielectric-coated groove. This stylus, along with the metalization and dielectric, acts as a capacitor. Capacitance variations in the groove, which correspond to the recorded video information, are then detected electronically to recover the video information.
Video discs produced, for example, in accordance with the above-described method contain closely spaced, signal representative, topographic patterns in the spiral convolutions of the disc. These topographic patterns may have successive peaks spaced as closely as 1.5 microns apart. In order to form the relatively small, and closely spaced video-representative topography, it is necessary to provide a relatively vibration-free turntable upon which the photoresist-coated video disc is mounted when the video information is recorded thereon. It is further necessary to provide a relatively constant and flutter-free rotational velocity of the turntable during the recording process. A turntable having a relatively flutter-free and constant rotational velocity is necessary to prevent subjectively objectionable disturbances (e.g., image jitter) from appearing on the video monitor during playback of the embossed or pressed video disc. The problems of providing a relatively vibration-free, flutterless, constant velocity turntable are compounded by the necessity of operating the electron microscope and therefore the turntable in a vacuum atmosphere. Recording on the video disc in a vacuum atmosphere is necessary to allow the video modulated recording beam of the scanning electron microscope to impinge directly upon the photoresist coated disc without interference from any atmospheric substances.
Turntable rotation in a vacuum atmosphere may be implemented by an electric motor mounted within the vacuum chamber. Electric motors, however, have the undesirable quality in that heat and magnetic fields are typically generated by their electric circuits. In a vacuum atmosphere, heat generated by the electric motor may not be dissipated by convection, thus undesirably limiting the means for cooling the motor. Magnetic fields generated by the electric motor may further produce the undesirable effect of perturbing the electron beam. It is therefore undesirable to utilize an electric motor housed within the vacuum chamber. Utilization of an electric motor mounted external to the vacuum chamber to drive the turntable presents other difficulties. Such an externally mounted electric motor is required to transmit rotational motion through a vacuum seal in the chamber, making the elimination of vibration and other disturbances difficult.
For reasons mentioned above, it is desirable to ameliorate all disturbances affecting smooth turntable rotation. Other disturbances, for example, those associated with the horizontal translation of the turntable, are less critical than those affecting rotation and need not be as precisely controlled. It is, therefore, particularly desirable to provide a rotational driving means that is free of external coupling, produces no electrical fields, and provides an accurate means of rotating a turntable in a vacuum chamber.